The invention relates to a rolling bearing steel from the 1C-1,5 Cr type series. Such steel comprises, generally, the following composition:
Carbon: 0.85-1.10 by weight %
Silicon: 0.005-0.6 by weight %
Manganese: 0.005-0.80 by weight %
Chromium 1.25-2.05 by weight %
Nickel: 0.35 max by weight %
Molybdenum 0.36 max by weight %
Balance Fe and usual impurities.
Steels comprising this composition are widely used in the production of rolling bearing components. Starting from a ferritic structure, the steel is subjected to an austenizing heat treatment, after which a quenching treatment results in a final component having a martensitic surface structure. This martensitic structure is relatively hard and has good basic properties. Applications for which the rolling contact fatigue life and toughness are of interest, carburised steels are used. The carburising steels and heat treatments are more costly, and the related heat treatments are generally much more complicated.
EP 0896068A1 discloses a method of bainite hardening of a bearing steel. To that end the starting material is in ferritic condition. The ferric steel is austenized and then quencehed, resulting in a bainitic final structure.
The invention aims to obtain a steel with improved properties, and more particularly, having improved rolling contact fatigue and good toughness properties.
According to the invention, this goal is realized by subjecting the ferritic steel to a deformation. This deformation can either be warm or cold. If warm deformation is used, preferably a deformation in the ferrite phase, i.e., of a temperature below 700xc2x0 C., takes place. During warm deformation the dislocation cells obtained during deformation recover to form fine sub grains, during heating to the hardening temperature, and therefore a finer structure is obtained as a result of the applied lower bainitic hardening process.
Preferably, the steel is subjected to shaping by rolling. More preferable, if a ring has to be produced as a rolling bearing component starting from a tube, cold deformation is effected, during which the ring itself is produced from the tube with less metal cutting operations. This means that there is less material loss. It has been found that if cold rolling is used, the austenite start and the austenite finish temperature will decrease, i.e., the transformation from ferrite to austenite will be at a lower temperature level, and will be more complete at the same temperature level. The bainitic transformation time is preferably at least 180 minutes. Except from lowering the austenizing temperatures by rolling, and more particular cold rolling, the martensite start temperature is also lowered by about 30xc2x0 C. and well below 250xc2x0 C. Generally, the microstructure shows a much-refined grain. Preferably, the bainite comprises lower bainite which results in an extra extension of the service life of rolling bearing components made from such steel.
It is, of course, possible to start from another article as a tube at deformation. For example, parts or rings are mentioned, possibly followed by a pri shaping process (turning, milling). Cold forming can comprise rolling, forging, shaping and so on.
The ferrite subgrain boundaries are probably austenite nuclination sites at the intersections with spheroidal carbides, which result in refinement of the austenite grain size compared to undeformed 1C-1.5 Cr austenized under the same conditions.
The steel used is preferably relatively pure, i.e., comprises 9 ppm oxygen max, 0.004 wt % sulphur max. 15 ppm titanium max and 0.015 wt % phosphorus max.
To show the beneficial effect of cold rolling relative to hot rolling when producing a rolling bearing ring, comparative tests have been conducted. Apart from either the hot rolling or cold rolling, the heat treatment in both samples has been exactly the same. It has been shown that, in a spherical roller bearing, the relative L10 life of the hot rolled variant is 106 with 95 upper and lower confidence interval of 52-157 million revolutions.
Under the same test condition, a cold rolled ring had an L10 life of more than 294 million revolutions. Although not essential for the scope of protection for the invention, it is meant that, because of cold rolling of the ferritic matrix, dislocations recover to cells resulting in sub grain formation. This sub grain formation will lead to finer austenite. Quenching starts from the temperature above martensite start.